In a home appliance involving moving parts it is desirable that the motion quickly stop after the electric power is turned off to avoid possible injury to the user. In those appliances driven by fractional horsepower induction motors, the rotor of the motor is relatively heavy and moreover the coasting motor is very quiet as compared to a fractional Horsepower universal-type electric motor having a commutator and brushes, so that the user is not audibly alerted to the continuing coasting motion of the induction motor and the associated driven parts. This quiet coasting particularly occurs in induction-motor-driven food processors.
Such food processors are kitchen appliances utilizing a variety of interchangeable rotary tools, as for example, knives, blades, cutting discs, and rasping discs for performing such operations as cutting, slicing, mixing, blending, grating, shredding, chopping and pureeing, etc.
Known food processors generally include a supporting base structure having an induction motor encased therein, a work bowl adapted to be seated on the supporting base with a driveable shaft in the work bowl. The specific rotary tool needed for a desired food processing application is removably mounted on the shaft. When the motor is actuated, such a tool spins rapidly within the work bowl. A cover having an opening is removably mounted to the top of the work bowl. Food to be processed is inserted through the opening in the cover and into the work bowl to be processed by the rotary tool.
Fractional Horsepower single-phase powered induction motors of the type generally referred to above are used in many such food processors. Such induction motors are energized by single-phase electric power and so they include a main winding and an auxiliary starter winding. There is a phase-shifting electric component connected in series with the auxiliary winding, such component usually being a capacitor, and the starting winding is energized briefly to start rotation of the motor. The specific tools used by the appliance are rotated at relatively high speeds during operation to perform their required functions.
In known food processors, the induction-motor-driven rotary tool can revolve at speeds of approximately 1,800 RPM. Thus, it is important that the processor include a braking system which will minimize coasting of the rotary tool after the motor has been de-energized. Such braking systems provide the user with quick access to the processed food in the work bowl and also minimize the possibility of injury to the hands of a user if inserted into the bowl prematurely or inadvertently.
Home appliances such as food processors are often equipped with protective switches which automatically shut off the power when a cover is removed from the zone where the tool is located. Nevertheless, the coast-down time may exceed the time taken to remove the cover and lay it aside. Thus a quickly moving user might be able to remove the cover and insert fingers or hand into the tool zone with consequent injury from the still coasting tool.
In known braking systems for induction motors, as for example the one illustrated in U.S. Pat. No. 2,613,342, a starting capacitor of the induction motor is electrically coupled to the windings of the induction motor to produce an electromagnetic braking effect within the motor when a control switch is in the "off" position, the braking effect is produced by converting the rotational mechanical energy into electrical energy which is quickly dissipated as heat.
If the induction motor is placed under a heavy load it slows down, thereby drawing on increased current, and the starting switch becomes closed placing the starting winding and starting capacitor in circuit. Under such circumstances, if the starting switch is suddenly turned OFF, damage can result to the ON-OFF switch in a circuit as shown in FIG. 1 of the Thompson patent, because the starting and running windings both suddenly become connected effectively in parallel across the switch contacts. Their sudden deenergization under such circumstances often produces severe electric arcing within the switch in my experience. When a fast-acting ON-OFF switch is used in a circuit as shown in FIG. 1 of the Thompson patent I consider it to be a "switch killer" whenever the induction motor is turned OFF under such a heavy load that the starting switch is closed.
In practice, when the motor is turned OFF under no load with a conventional size starting capacitor, it has been found that in a braking system as described above, the braking effect afforded by the starting capacitor of the induction motor is not so great as desired. To overcome this difficulty, it has been proposed electrically to connect a second capacitor into the braking circuit. For example, please see FIG. 2 of the previously mentioned U.S. Pat. No. 2,613,342--Thompson.
However, providing a braking circuit with the above described second capacitor has resulted in difficulties. For example, if the second capacitor is inadvertently connected across the line voltage, it is likely to be destroyed. In said patent there is employed a double-pole, double-throw (DPDT) switch which is inherently slow-acting because there is an intermediate position (namely, the position as shown in FIG. 2 of said patent) through which the switch arms must be moved and during which interval the switch is wide open so that no braking at all is occurring. In Column 2, lines 8-14, Thompson states:
"The single pole double throw switch 12 may be replaced by a different switch, for example a drum or magnetic switch, which allows instantaneous shifting from running to braking positions and which has no `coast` position such as is present in switch 12 when it is closed to neither terminal."
This text implicitly recognizes the slow action of the switching circuit shown, and drum or magnetic switches are complex, not being suitable for a home appliance such as a food processor. Moreover, Thompson does not suggest the use of drum or magnetic switches to replace the DPDT switch in FIG. 2, which is the circuit actually containing a second capacitor.
Although Thompson shows a second capacitor in FIG. 2 this added capacitor 16 is actually useless during the major portion of the coasting time, because it soon becomes short-circuited by the starting switch 7. Since such a starting switch normally closes slightly below the normal running speed of the motor, the added capacitor 16 is rendered useless during the major portion of the time when the motor is being slowed down. Consequently, only the starting capacitor alone is of much effect in the Thompson circuit. This lack of much utility in the added capacitor is tacitly admitted since only the capacitance value of the starting capacitor is set forth in the table in Column 3, and no value is given for the added capacitor.